1. Field of the Invention
This invention relates to a method for subtractive workpiece processing and more particularly to a method for controlling the subtractive material processing so that maintaining a given tolerance of material thickness to be removed is ensured.
2. Description of the Prior Art
Subtractive workpiece processing frequently involves the removing of a predetermined layer thickness with maximum precision, and the terminating of the process in time, particularly in connection with etching and other kinds of subtractive processing of electrical components, in particular semiconductor and ceramic components to whose precise form and dimensional stability very high demands are made. These methods can involve processes like sputter etching, reactive ion etching, or plasma etching in connection with techniques like photolithography, but also cutting, grinding and other methods by which material is removed from a workpiece.
For some materials, as e.g. sintered ceramic substances the etching speed is extremely low, e.g. in the order of 200 .ANG./min. corresponding to 20 nm/min. so that for a structural depth of for instance 12 .mu.m etch times of approximately 10 hours are required. In the case of sputter etching, the etch rates are determined by various sputter parameters, as gas pressure, gas composition, gas flow, etc. Specific variations of these parameters complicate the maintaining of the very narrow admissible tolerances, which in the above given example can be approximately .+-.1 .mu.m, so that even if the calculated etch time is strictly observed there is a considerable risk of the tolerances being exceeded, and of the workpiece becoming useless. The consequences are particularly serious in those cases where in one single etching process a high number of workpieces are processed simultaneously. This kind of process frequently involves several hundreds of parts which are simultaneously exposed to an etching process.
Although it is possible to add a supplementary etching process if the predetermined minimum depth of the layer to be removed has not yet been reached, the etching rates vary considerably e.g. at the beginning of a sputtering process until the process parameters have reached stability. It is therefore complicated also in this particular case precisely to maintain the desired etching depth.
Consequently, it has to be made sure that in subtractive processes the desired removal depth can be maintained as precisely as possible, which in view of the above specified problems is possible only with a reliable continuous supervision of the respective depth reached, and by a corresponding control of the process. This control should be effected in situ, e.g. during the actual process, without the workpiece being detached, and if possible without the process being interrupted.
It is known to monitor the etching rate of semiconductor elements by continuously observing the thickness of the element, or the thickness decrease during subtracting, by means of infrared radiation whose rays reflected by the two surfaces are made to interfere (IBM Technical Disclosure Bulletin, Vol. 20, No. 6, pp. 2268/2269). The measuring is executed by an infrared detector, and the thickness decrease can be calculated taking into consideration the known refraction index of the material. This process can be executed in situ, but it requires a complex structure and furthermore a relatively long processing period.
Another method for determining the depth of workpieces, e.g. thin film circuit elements with different layers of material, consists in analyzing the respective crater form as a function of the subtractive processing, e.g. sputtering (IBM Technical Disclosure Bulletin, Vol. 21, No. 2, p. 672). However, workpiece layers of different hardness have to be considered here.
In another method of measuring a recess in a workpiece a microscope is used which is directed at an angle of 45.degree. onto the vertical wall of the recess. If the workpiece is horizontally displayed relative to the microscope, or vice versa from a microscope target in the recess wall to the next target, the corresponding vertical distance corresponds to the displacement distance since both distances correspond to the two legs of an isosceles triangle. However, this method described in IBM Technical Disclosure Bulletin, Vol. 18, No. 7, p. 2069 requires a recess surface that is precisely vertical, i.e. that extends at right angles to the surface of the workpiece, as well as the means for microscopic observation.